This invention relates to the manufacture of fibrous webs, and especially to improvements in a method and apparatus for laying down a fibrous web from a foam-fiber furnish.
In the manufacture of fibrous webs, such as paper, from a foam-fiber furnish deposited on a forming wire from the slice of a conventional foam-forming headbox, it has been found difficult to maintain a desired random orientation of fibers ensuring optimum MD/CD tensile strength of the formed sheet at the preferred, relatively high wire speeds associated with papermaking. Efforts at achieving a desired fiber orientation have involved delivering the foam-fiber furnish to the slice, immediately upon creation of the furnish.
Apparatus and methods are found in the prior art for depositing foam-fiber furnishes on forming wires, immediately upon its formation, to achieve a range of MD/CD ratios. U.S. Pat. Nos. 3,798,122 and 3,837,999 are exemplary of prior art teaching of foam-fiber furnish deposition in the manufacture of fibrous webs of predetermined MD/CD ratios through control of fiber orientation. Of these patents:
U.S. Pat. No. 3,798,122 discloses a mechanical foam generator for a foamable liquid-fiber furnish, wherein the foamed furnish having an air content from about 56% to about 67% is immediately discharged from a mechanical foam generator through a slice onto a forming wire moving at speeds in a range of from about 300 to about 1500 feet per minute, in achievement of a desired random orientation of fibers in the formed web; and
U.S. Pat. No. 3,837,999 discloses control of fiber orientation in a foam-fiber furnish, having an air content from about 65% to about 75%, by varying the wetted perimeter of a nozzle from which the foam furnish, immediately upon its formation in a mechanical generator, is discharged onto a forming wire moving at relatively low speed in a range of from about 90 to about 120 feet per minute.
U.S. Pat. No. 3,937,273 discloses foaming in a headbox per se by introduction of air into a foamable paper-making furnish in the headbox, to generate foam and create turbulence for preventing flocculation.
The following U.S. patents, while not concerned with foam-fiber furnishes, are exemplary of art relating to agitation of liquid-fiber furnish immediately prior to its deposition on a forming wire:
U.S. Pat. Nos. 3,954,558 and 4,021,296 disclose apparatus for feeding liquid-fiber furnishes, including convoluted turbulencegenerating headboxes; and
U.S. Pat. Nos. 3,092,540, 3,846,230, and 3,201,306 disclose headbox apparatus provided with abutment portions against which fluid-fiber furnish is impacted to create turbulence.
Further art relating to papermaking from a foam-fiber furnish includes U.S. Pat. No. 3,938,782 that discloses feeding a mixture of air, surface active agent, fibers, and liquid through foaming nozzles that both foam the liquid and randomly orient the fibers. The foam-fiber furnish is, however, fed to the headbox channel through relatively long reaction tubes which disadvantageously tend to unidirectionally orient the fibers as they are presented for flow through the headbox slice and deposited on a forming wire.
While it will be appreciated that the present disclosure does have in common with the disclosure of the '782 patent a non-mechanical foam generator for a foam having a volume percentage of gas in the range of from about 55 to about 75 percent to achieve uniform dispersion of fibers in a foam-fiber furnish, the presently claimed invention has as a general objective, as an improvement over art exemplified by the '782 and the '122 patents in particular, the provision of the novel combination of a headbox with in-line foam generating nozzles for achieving random orientation of fibers in a foam-fiber furnish as it is deposited or spread by the headbox slice, at an efflux ratio (deposition speed/wire speed) of about 1.25, onto a forming wire moving from about 1500 to about 4000 feet per minute, to form a fibrous web having an improved MD/CD tensile strength ratio.
The present invention takes into account teaching based on fluid dynamics theory that fluid flow will be laminar or turbulent depending upon a value of the Reynolds number as defined by the following equation: EQU N.sub.Re =.rho.Vd/u
Where:
N.sub.Re =the Reynolds number PA1 .rho.=fluid density PA1 V=flow velocity PA1 d=flow channel dimension PA1 u=fluid viscosity
For foam having the consistency and makeup contemplated by the present invention, and hereinbelow to be more fully described, a Reynolds number of 10,000 defines the point above which foam flow ceases to be laminar and becomes turbulent.
Since the foam has about one third the density of water, while having from about 10 to about 35 times the viscosity of water, it will be appreciated from the above equation that it is much more difficult to create turbulent flow of foam. Foam, however, exhibits pseudo-plastic behavior, so that when it is subjected to shear of a sufficient rate, the apparent viscosity is reduced and its flow is most susceptible of becoming turbulent.
It will be appreciated from what follows that the present invention uses to maximum advantage this principle in the creation of turbulent flow of a foam-fiber furnish to obtain random fiber orientation.